1. Field of the Invention
This invention relates to single lens reflex cameras, and more particularly to a movable mirror with an auxiliary mirror suited to electronic still cameras.
2. Description of the Related Art
Of the single lens reflex cameras hitherto developed, Japanese Laid-Open Patent Application No. SHO 60-60611 discloses one in which as shown in FIG. 1, the movable mirror for reflecting the image bearing beam from the photographic objective to the finder system is provided with a half-reflective area in one surface thereof, and an auxiliary mirror is provided in axial alignment with the half-reflective area at such an angle therewith that the transmitted part of the image bearing beam through the half-reflective area is reflected to an automatic focus detector or a light meter positioned on the bottom of the mirror box. The general mechanism for operating such main and auxiliary mirrors is explained here by reference to FIG. 1 where 1 is a camera body. The main or tiltable mirror 2 when in viewing position reflects the image bearing beam upward to the finder system. A holding frame 3 for the mirror 2 is pivotal about a shaft 4. A stopper 5 limits the counterclockwise movement of the holding frame 3 to define the viewing position of the tiltable mirror 2. A central portion of the area of the tiltable mirror 2 is made half-reflective. Light emerging rearwardly of the central portion is reflected by an auxiliary mirror 6 tiltably provided on the tiltable mirror 2 to the automatic focus detector 7 on the bottom of the mirror box.
A holder 9 for the auxiliary mirror 6 is pivotally mounted on a shaft 10, and is urged by a spring (not shown) to turn in a direction indicated by arrow B. A stopper 11 on a camera casing limits the movement of the holder 9 in the direction B. A cam 12 is provided on an end portion of the holder 9 and arranged upon upward movement of the tiltable mirror 2 to turn the holder 9 in the opposite direction to the direction B as the cam 12 moves in contact with a pin (not shown) fixed to the camera body 1. In the non-viewing position, the holder 9 with the auxiliary mirror 6 is light-tightly folded on the holding frame 3.
This mirror arrangement for the focus detector shown in FIG. 1 is desirable when the photographic material is the so-called silver halide film, and when the format is 35 mm in size.
In a camera of smaller format such as a so-called electronic still camera, whose format has its two sides each equal to about a quarter of that of the format of the 35 mm single lens reflex camera, however, as its picture frame size decreases, the size of the tiltable mirror and auxiliary mirror is also decreased, and the size of the auxiliary mirror provided on the reduced-in-size movable mirror is reduced. Therefore, the requirement for the tolerance of the positions the movable mirror and auxiliary mirror take when they return from the non-viewing position should be extremely rigorous. In other words, the auxiliary mirror must be folded while the movable mirror retracts from the path of the image bearing beam. Thus, as the size of the movable mirror and the auxiliary mirror decreases, the difficulty of insuring high precision accuracy of their linking mechanism increases rapidly.
Generally, it is difficult to attain the high precision of a driving mechanism to precisely maintain the stability of the reflecting direction characteristic of the movable mirror or the auxiliary mirror. Therefore, where a focus detection or light measurement is effected using a light bundle reflected by the auxiliary mirror, it is very difficult to effect the focus detection or light measurement with high precision.
Also, in electronic still cameras and others, the picture frame size is a quarter of the 35 mm size, and the focal length of the photographic lens is about 1/4 times shorter than 35 mm camera. The back focal distance also becomes shorter so that the distance from the lens mount to the image pickup device cannot be made long. Moreover, the tiltable mirror which is to be provided behind the photographic lens is positioned slightly away from the image pickup device such as CCD or the like. Further, for the image pickup device such as CCD or the like, various photographing optical members such as a quartz plate, an infrared rays cut filter, a cover glass and a color filter are arranged between the tiltable mirror and the image pickup device. For this reason, when the auxiliary mirror is arranged such that the image bearing beam, after reflection of the auxiliary mirror, emerges progressively nearer to the film than the point of incidence of it on the auxiliary mirror, as in the single lens reflex camera of FIG. 1 whose film size is 35 mm and silver halide is applied to the electronic still camera or the like, the above-described optical member such as the filters and the image pickup device become obstracles and, at the same time, the bottom portion of the mirror box becomes complicated. Because the bottom portion of the mirror box must be expanded, the camera as a whole becomes larger in size.
Also, on the assumption that, for example, automatic focus detection is performed by utilizing that part of the image bearing beam which has passed through the half-reflective area of the tiltable mirror in the form of the reflected light beam by the conventional type auxiliary mirror, then, since the length of the optical path from the tiltable mirror to the image pickup device becomes relatively long, when the length of the optical path from the tiltable mirror to the automatic focus detector is taken at the same value, it becomes very difficult in terms of space to arrange the focus detector without blocking any part of the image bearing beam to the image pickup device.
Further, the image forming system including the above-described various filters in between the photographic lens for the electronic camera and the image pickup device such as CCD or the like differs in optical performance from image forming systems that exclude optical members equivalent to these various filters from between the lens and, for example, the focus detector. Particularly when the focus detection is performed by measuring the degree of sharpness of an image of an object to be photographed, that difference gives rise to the drawback of lowering the accuracy of focus detection.